As known in the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. When employing twin-wire draw, a group of drying cylinders comprises two closed (endless) wires, fabrics or belts which press the web, one from above and the other one from below, against heated cylinder faces of drying cylinders arranged in rows. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering and may cause web breaks, in particular in drying stages in which the web is still relatively moist and, therefore has a low strength. For this reason, in recent years, ever increasing use has been made of the single-wire draw in which each group of drying cylinders includes only a single closed (endless) drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, whereas on the reversing cylinders or rolls between the drying cylinders, the web remains at the side of the outside curve and is subjected to negative pressure as it runs over the reversing cylinders in order to maintain the web on the wire. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the wire loop.
In a so-called normal dryer group with single-wire draw, known in the prior art, the heated drying cylinders are placed in an upper row and the reversing cylinders or rolls are placed in a lower row below the upper row of drying cylinders, which rows are typically horizontal and parallel to one another. In a so-called inverted dryer group, the heated drying cylinders are placed in a lower row and the reversing cylinders or rolls are placed in an upper row above the lower row of drying cylinders. In the following, when the terms "normal (dryer) group" and "inverted (dryer) group" are used, what is meant is expressly groups with single-wire draw in multi-cylinder dryers, of the type mentioned above.
In prior art drying devices, the drying wire and the paper web come from a preceding drying element, for example a contact-drying suction cylinder, onto a reversing suction cylinder or equivalent as a joint straight run. In this case, a closing wedge space is formed between the drying wire and the face of the reversing suction cylinder, which wedge space is, in the following, also called an inlet nip. The moving drying wire and the face of the reversing suction cylinder tend to induce a pressure in the wedge space. This produces a difference in pressure acting upon the paper web placed on support of the drying wire, which difference in pressure attempts to separate the paper web from the drying wire and causes problems of runnability, wrinkles, and even web breaks. On the other hand, in order to improve the efficiency of dryer sections, there is a need to use ever more compact dryer sections in which the contact-drying cylinders and suction cylinders are placed as close to one another as possible. All of these factors, together with the increasing web speeds, increase the problems of the pressure induced in the inlet nip.
As known in the prior art, in single-wire draw, the transfer of the paper web from a reversing suction cylinder onto a contact-drying cylinder usually takes place so that the web is supported on the wire by means of a field of vacuum produced by a blow box or equivalent web-adhering arrangement. The current assignee markets blow boxes with the trade mark UNO-RUN-BLOW-BOX.TM., and with respect to the principle of operation of such blow boxes, reference is made to the current assignee's U.S. Pat. No. 4,516,330 (corresponding to Finnish Patent No. 65,460), incorporated by reference herein. By means of these blow box constructions, the paper web can be passed from a drying cylinder onto a reversing suction cylinder at the paper machine running speeds of less than about 1600 meters per minute currently in use.
In the prior art constructions, attempts have also been made to eliminate the problem related to the pressure peak induced in the inlet nip by means of suction in rolls, suction in sectors of rolls, and by means of suction boxes of different types as well as by means of combinations of rolls and suction boxes. However, by means of such arrangements, it has, however, not been possible to eliminate the pressure peak formed in the inlet nip.
In the prior art, it is known that, in order to hold the web in contact with the face of a reversing cylinder or roll, a vacuum is sufficient that overcomes the force arising from the centrifugal force. This required pressure can be calculated by means of the equation: ##EQU1## wherein:
p=pressure (N/sq.m),
Pv=basis weight of web (kg/sq.m),
U=circumferential speed (meters per second),
r=radius of cylinder (meters).
Depending on the running speed of the machine and the weight of the web, the requirement of pressure is of an order of from about 100 Pa to about 200 Pa. Thus, a problem has been the control of the pressure formed in the inlet nip.
It has also been possible to reduce the detrimental pressure induced in the inlet nip by means of grooves and bores on the roll. However, by means of these structural modifications, it has not been possible to provide a substantial solution for the problems arising from the pressure induced in the inlet nip.
With respect to the prior art related to the present invention, reference is made to U.S. Pat. No. 3,259,961 which describes a cell construction that produces a pressure in the inlet nip because it comprises walls arranged perpendicular to the movement of rotation.
Further, reference is made, for example, to the current assignee's U.S. Pat. Nos. 4,202,113 and 4,441,263 (both of which are incorporated by reference herein) and Finnish Patent Nos. 93,876 (corresponding to U.S. Pat. No. 5,553,393, incorporated by reference herein) and 83,680 (corresponding to U.S. Pat. Nos. 5,022,163 and 5,172,491, incorporated by reference herein), which describe devices in dryer section of paper machine but which, do not suggest a fully adequate solution for the problems arising from the pressure formed in the inlet nip.